Typical AFC systems for television receivers detect the frequency of the intermediate frequency (IF) signal and develop an error signal dependent upon the frequency deviation of the IF video carrier from the desired intermediate frequency. The error signal is applied to the radio frequency (RF) tuner oscillator to correct or adjust the oscillator frequency, and hence, the IF video carrier, toward the desired frequency. If the tuner oscillator frequency falls relatively close to the desired frequency, an appropriately designed narrow band AFC system will normally lock on the IF video carrier. Unfortunately, tuners are mechanically switched devices that operate at high frequencies and assuring that the frequency of the tuner oscillator always falls within predetermined narrow limits is prohibitively expensive and complex particularly when both the UHF and VHF bands are considered. While wide band AFC systems have been used with more or less adequate performance, a persistent problem has been the capability of the AFC system to lock onto the co-channel sound carrier. Prior art systems for preventing locking on the sound carrier have generally involved relatively complex, and hence, unduly expensive circuitry, erratic or unpredictable performance, and other deleterious effects.
RF tuners in common use generally fall into two broad catagories: (1) tuners which use passive components such as coils and/or capacitors as frequency determining elements, and (2) tuners which use voltage controlled capacitances such as varactor diodes as the frequency determining elements. While other varieties or combinations of frequency determining elements can be used, the elements for determining the oscillator frequency generally has been of one of the two types mentioned. The mechanism for correcting the oscillator frequency will necessarily vary depending upon the type of frequency determining elements used. For example, varactor diodes have a non-linear characteristic which results in a change in sensitivity depending upon the frequency, and hence, the applied voltage at which the tuner is operating. Accordingly, the error voltage from the AFC circuit will result in different pull-in and hold-in ranges for different channels. Furthermore, it is often desirable to use the same AFC circuit for receivers with tuners of various types, however, the varying characteristics and responses to correction voltages of the different types of tuners have heretofore required substantially dissimilar AFC circuits. In yet other arrangements, it is desirable to use a UHF tuner with voltage controlled capacitances and a VHF tuner with passive frequency determining elements, or vice versa, in the same receivers while incorporating only one AFC circuit for both tuners.